Masonry Structures

Structural Investigation and Assessment

Ian Hume

There are essentially two approaches
to the investigation and assessment of
masonry structures. The first involves
basic, low-tech methods, while the second
uses much more sophisticated, and therefore
more expensive, techniques. This article
looks mainly at the less expensive methods
available. It also looks briefly at the questions
raised by investigative work and where to go
for further help.

THE AIM OF INVESTIGATION

Appropriate investigation and assessment
will help ensure that repairs to historic
masonry are carried out where necessary and
that unnecessary work is not undertaken,
minimising the loss of original fabric and
ensuring that money is wisely spent. Not
everything is as bad as it sometimes seems but,
conversely, structures which might initially
seem sound can have hidden problems.

Considering the condition of individual
stones or bricks to estimate their remaining
lifespan is a fairly straightforward part of
masonry investigation and assessment. The
condition of the small elements that make
up a masonry wall rarely affects its overall
condition. Figure 2, which shows stones at
Dunstanburgh Castle in Northumberland,
illustrates this point. The much decayed stone in Figure 2a, below, cannot have much life left
in it, but it could probably erode away entirely
without threatening the stability of the wall.
However, if a whole series of stones decays
then problems will arise, which is why the
repair shown in Figure 2b was carried out.

The same argument does not apply
where loads are greater. In a column,
for example, the serious decay of one
stone can affect the stability of the whole
column (figure 3), with potentially serious
consequences for the entire building.

SIMPLE INVESTIGATIVE TECHNIQUES

The basic techniques described below are used
for the initial investigation and assessment
of most masonry structures. Further, more
sophisticated assessment techniques may
also be useful, but when the assessment is of
a local church, a vernacular house in private
ownership or a small commercial property,
the funds available may only be sufficient for
the basic techniques.

1

Historic research

Historic research
Assessing the date of the structure is a
sensible place to start as this will give clues
to the likely construction of the masonry. For
example, medieval masonry is likely to be very
thick but may consist of two well constructed
skins of cut masonry or random rubble with a
rubble and mortar core. The wall core might
be well built but, to quote the Roman architect
Marcus Vitruvius Pollio, it is more likely to
include ‘a lot of broken stone and mortar
thrown in anyhow’.

Georgian and Victorian walls may not
be as well constructed as they might at first
appear because they may contain ‘snap
headers’ (bricks with the short face visible as
if bonding inner and outer leafs, but which
are in fact half-bricks). This was a cost-cutting
measure which enabled cheaper bricks to
be used for the whole of the inner leaf, at
the expense of reducing the effectiveness of
bonding between the face and the main body
of the masonry. If the building is a substantial
one built around 1800 it may contain bonding
timbers, which are prone to decay.

2

Visual inspection

This will involve looking for cracks, signs of
settlement, leans and other distortions as well
as inspecting the condition of the individual
stones that make up the walls or columns. It is
important to look at and along the face of the
wall seeking out signs of bowing or bulging.
This may indicate movement of the body of the whole wall or, more likely, ‘face separation’
where the outer skin parts company with the
core, often known as delamination.

Tapping (not hitting) the face of the structure
with a lump hammer or similar tool may
reveal evidence of deterioration. If the wall is
sound there should be a good ‘ring’ but a dull
thud can indicate delamination. However,
like so many things in conservation this sort
of test relies on experience. The sound of
a hammer striking a wall can also depend
on the state of the mortar and the size of
the individual brick or stone elements. This
particularly affects the sound of a flint wall,
where there is a much higher ratio of mortar
to stone than in a brick or stone wall. Being
a very hard material set in soft mortar it is
sometimes possible to inadvertently drive
flints into the wall. The wall shown in figure 5
looked very sound and a hammer inspection
confirmed this but experience and the
presence of a bulge suggested otherwise.

4

Monitoring

Where there are fractures and signs of
movement it is vital to ascertain whether
these are active and ongoing movements
or merely indications of historic problems
that settled down long ago and only need
some local pointing. The structure may
need to be monitored by a structural
engineer experienced in conservation to
finally establish whether or not it is moving.
Figure 4 shows a classic example of historic
movement. This brick gable wall belongs to a
Georgian house but the major crack does not
pass through the Victorian plaster cornice,
indicating that the crack existed before the
ceiling was installed and has not moved since.

5

Excavation

Removal of small elements of the wall allows
the assessor to properly consider the condition
of the core of the wall. This may just involve
cutting out a few bed joints here and there
(using a pointing chisel or quirk) or perhaps
carefully removing the occasional brick or
stone. It is important to remember that this
sort of intrusive inspection may well require
listed building consent.

6

Borescope inspection

Consider using a borescope (endoscope)
to inspect the core of the wall. These
instruments, which come in a variety of
lengths and may be rigid or flexible, can be
inserted into existing cracks in the wall or it
may be necessary to purpose-drill small holes.

MORE SOPHISTICATED,
NON-DESTRUCTIVE METHODS

Figure 3 A column threatened by the corrosion
expansion of iron cramps: no serious masonry decay
but more than enough to threaten the stability of the
column and, in turn, that of much of the structure

There is a range of more sophisticated
(and therefore more expensive and time-consuming)
methods of investigating masonry
structures. These methods are not examined
in detail here as they are highly technical
and the outcome of any survey using them
can depend on the type of structure being
investigated. It is important to seek further
information, initially by searching the internet
(see Further Information below) and then,
most importantly, by discussing the work with
experienced practitioners before embarking
on an expensive and possibly futile procedure.
Will the method proposed work on the type
of structure concerned? Are there better
methods available? What access is needed?
How much will it cost? These questions will
need to be answered and there may be other
concerns to be addressed that are specific to
each technique.

Nevertheless, the results of good non-destructive
testing can be extremely useful.
For example, It is often useful to ascertain
whether there is any metal in the structure
either in the form of cramps (these will usually
give themselves away by breaking the corners
off stones due to the expansion of rust) or
buried metal ties (see figure 6). The latter
may be present as wrought iron rings around
towers and domes to resist outward thrust or
as hoop iron used as bed joint reinforcement.

OPTIONS

Radar An echo-sounding method which
uses radar (radio waves) to detect voids or
buried metals, ground penetrating radar
(GPR) offers greater penetration than
metal detectors but is more expensive.
GPR can also determine something of
the corrosion level, whereas a metal
detector will not respond successfully
to a pile of corrosion product which is

perhaps all that remains of buried metal.

Ultrasonics An echo-sounding method
which uses ultrasound, this technology is
also used for seeking voids in masonry.

Thermal imaging The use of thermal
imaging cameras can highlight
delamination and moisture penetration due
to fine variations in surface temperature,
and may be carried out remotely.

Video surveys A wide range of video
capture services are available for remote
inspections and detailed surveys, from
drain surveys to aerial inspections.
Video survey cameras have been
mounted on dirigibles and helicopters,
wheeled robots and telescopic masts,
and hand-held cameras can be used by steeplejacks and abseilers to record
otherwise inaccessible faces of buildings.

It may well be useful to have the benefit
of input from more than one method to
supplement what your eyes and experience
tell you.

HAVING USED THESE METHODS,
WHAT THEN?

The next step is to determine why the
structure has moved or deteriorated in the
way it has, as there is little point in repairing
a defect if the cause is not addressed. In the
case of structural movement, typical issues to
consider are outlined below.

Which part of the structure is settling
and which is essentially stable?

Could the cause be ground related? This
may seem unlikely in an old building but
movement can be related to inadequate
foundations, recent changes in water table
levels or the root systems of nearby trees.

Does the whole of the wall lean from
foundation level or does it emerge from the
ground essentially plumb but then curve?

Is it roof-thrust? Has there been
any impact, storm or bomb
damage? Have nearby building
works affected the structure?

Has excessive water ingress weakened
the structure? Do the gutters leak?

What changes has the structure
undergone and were any of these so
poorly designed as to cause movement?

Have thermal and or climatic changes
affected the structure? Is there sulphate
attack or have flue gasses caused
expansion of mortar bed joints?
The list of possible causes of movement is
almost endless and no amount of detailed or
sophisticated investigation can identify all the
issues in every case, but efficient investigation
adds enormously to the body of evidence that
can provide the answers.

Figure 4 (above left): A classic example of old movement in a wall of a Georgian house. Figure 5 (top right): Face delamination of a flint wall that
seemed, on first inspection, to be in very sound
condition. Figure 6 (lower right): Metal ties embedded in stonework can cause
damage when they rust.

SOURCING EXPERTISE

It is clearly advisable to speak to architectural
and engineering consultants who specialise in
historic building work and who are accredited
in conservation, and to other specialist
investigation consultants. A directory
of building conservation professionals is
available at buildingconservation.com and
the conservation accreditation registers
(see Further Information) list accredited
architects, surveyors and structural/civil
engineers. These registers list practitioners
who have a proven track record in
conservation, have been judged by their peers
to have kept up-to-date with continuing
professional development and have the
right background of a sound conservation
philosophy.

A number of courses in the UK teach
the basic conservation skills needed by
craftsmen and professionals. There are several
excellent post-graduate conservation courses run by various universities
that cover the skills discussed in this short
article. More targeted short courses are also
provided by a small number of specialist
organisations such as West Dean College
in Sussex. The college
runs short courses on masonry work and
structural repairs to historic buildings that
cover aspects of investigation and assessment.
West Dean courses can also make use of a
‘ruinette’, built around 20 years ago using
an eclectic mix of medieval, Tudor and
Victorian masonry materials and techniques.
Trainees are given the opportunity to carry
out repairs such as pointing and grouting,
rebuilding and repairs without risking
damage to authentic historic fabric.